The disposal of infectious waste from hospitals and other medical establishments is a major problem. Indeed, the importance of proper and effective infectious waste disposal has become of greater concern in recent years, due to an increased awareness of health problems such as the AIDS epidemic. In part because of the AIDS epidemic, definitions of what constitutes "infectious waste" are being broadened. Consequently, the volume of infectious waste which must be disposed of is increasing. Accordingly, the need for a system or apparatus which will accomplish the safe, efficacious, and cost effective treatment of significant volumes of infectious waste for disposal is growing.
One method for decontaminating infectious waste involves incineration, wherein the waste is burned and the decontaminated ashes are properly disposed. An alternative treatment method is to disinfect the waste in a steam autoclave prior to waste disposal. While effective for their intended purposes, both incinerators and autoclaves present ancillary problems. Incinerators, for example, are difficult and costly to construct and are relatively expensive to maintain in an environmentally safe manner. Autoclaves too, present additional problems, such as odor, cost and operational complexity. Additionally, waste which has been disinfected by autoclaving typically requires further treatment procedures, such as incineration or shredding and granulation, prior to final disposition of the waste in such places as landfills.
With the above discussion in mind, alternative infectious waste treatment systems have been proposed to disinfect the waste in preparation for disposal. According to these proposals, a solid infectious waste is contacted with a disinfectant solution containing a chlorine compound to decontaminate the waste. The decontaminated waste may then be disposed in ordinary landfills.
Unfortunately, decontamination of waste using chlorine compounds presents certain technical complications. First, liquid disinfectant loses its disinfectant potency during prolonged storage. Thus, there is a need to use liquid disinfectant that is relatively "fresh" in order to achieve an acceptable degree of waste decontamination. Second, it is relatively difficult to ensure that an appropriate concentration of the disinfectant has contacted the waste during the treatment process. It is also important, however, to avoid applying too high a concentration of chlorine compound to the waste, in order to avoid undesirable results, such as corrosive effects and the release of toxic gases. Significant health risks are known to result from the discharge of chlorine to the environment.
The most commonly used disinfectant is sodium hypochlorite, typically as a one percent solution. The strength of the solution is dictated by the necessity of achieving a desired rate of bacteria kill in a given apparatus, resulting in a given rate of use of the disinfectant when operating at a given rate of throughput of waste. The use of a one percent solution results in the discharge of a significant amount of chlorine into the environment from the typical apparatus, either into the sewer or absorbed into the processed waste.
Because of its higher reactivity, chlorine dioxide is far more effective than sodium hypochlorite for the treatment of infectious waste. Chlorine dioxide also typically exists as a gas in solution, greatly enhancing the penetration of the disinfectant into the waste material. Chlorine dioxide can, if applied to properly granulated waste, achieve the necessary kill rate at a concentration of only about 50 ppm, or only about 0.005 percent, or 5 one-thousandths of the necessary concentration of sodium hypochlorite. Finally, the chlorine dioxide is far less environmentally persistent, rapidly disassociating into sodium chloride, water, and citric acid. When taking into account the rate of use of sodium hypochlorite in a typical process, and the required rate of use of properly applied chlorine dioxide to achieve the same kill rate, the sodium hypochlorite process results in the discharge to the environment of approximately ten thousand times as much of the treatment chemical. This means that the chlorine dioxide process results in the discharge to the environment of an amount of chlorine which is minuscule, compared to the amount of chlorine discharged by the sodium hypochlorite process.
Unfortunately, chlorine dioxide is very corrosive, highly unstable, and even explosive. It can not simply be substituted for sodium hypochlorite in a process. It must be used in an apparatus designed to properly generate and mix the chemical, and designed to properly granulate and handle the waste material to allow the use of a very low concentration of the chemical. Further, it is impractical to store chlorine dioxide, because it is an oxidizer and exposure to oxidizable impurities would consume a portion of the chlorine dioxide. Effective exclusion of oxidizable impurities from the storage tank would be difficult. Because of these difficulties, sodium hypochlorite is almost always used instead, even though it is less effective, and even though it results in increased chlorine contamination of the environment.
The present invention recognizes that liquid precursors of chlorine dioxide can be stored for relatively lengthy time periods without losing their potency. Further, these liquid precursors can be mixed to form chlorine dioxide immediately prior to injection into a waste stream, in a continuous process. Chlorine dioxide can be produced by the combination of sodium chlorite with a strong mineral acid, such as hydrochloric acid, or with a weak organic acid, such as citric acid. The use of a strong acid tends to produce high concentrations of chlorine dioxide more rapidly, and the process tends to be more difficult to control. Treatment of medical waste requires very low concentrations of chlorine dioxide produced in a slow and predictable manner. Therefore, the best choice for treatment of medical waste is the combination of sodium chlorite with a weak organic acid, such as citric acid. The resulting reaction is slow and predictable, and it is easily mediated by controlling the temperature of the solution.
The resulting solution can be used in a very low concentration to decontaminate infectious waste, if used in a system that mechanically reduces the particle size of the waste to the appropriate size. The present invention also recognizes the necessity for the correct interaction of certain critical structural features in the waste processing apparatus, to achieve the necessary intimate contact between the low concentration of chlorine dioxide and the waste material, and to properly handle the waste material to allow the conservative use of the chlorine dioxide.
Accordingly, it is an object of the present invention to provide a system for waste treatment in which chlorine dioxide precursors are appropriately mixed and then immediately blended with infectious waste to decontaminate the waste, while preventing excessive decomposition of the disinfectant, and while preventing any explosion hazard. Another object of the present invention is to provide a system for waste treatment which results in the reduction of waste particle size to an appropriate size to allow effective use of the disinfectant in a low concentration, while preventing clogging of the waste stream and while maximizing the recycling of the disinfectant. Finally, it is an object of the present invention to provide a system for waste treatment which is relatively easy and comparatively cost-effective to implement.